Keeping print bed level (melamine bed)

I have a really vexing problem with my i3 rework: seems like I need to readjust the bed after every print. Sometimes I'll even level the bed perfectly, home it on z again, and see a different distance... My z endstop is essentially rock-solid in its position (rigidly supported from the bottom, can't slip), so it must be the bed itself. I read in an old post that somebody seems to have had that problem with a melamine bed: could that be the issue? I also notice that the bed seems to be slightly warped right now, which seems to hint that its lack of rigidity could be a problem. Basically, I'm trying to figure out if I have to buy an aluminum bed to get my print surface to stay level. If there's an easy fix I'm missing I'd gladly try that. Any thoughts?

Details:
I used to have the regular setup (m3 nut, bed, spring, heated bed, m3 screw goes through all that) but added another nut between the spring and bed and a stack of washers between the spring and heated bed. This made everything extremely rigid, which was nice, but difficult to adjust with the 2 nuts. The problem is that I still have to regularly readjust the bed because it won't keep a consistent distance even for one print. I also tried marking the screws with pencil and watching them during a print: it doesn't seem like they move much during the print, so I doubt they're the reason for changing bed to nozzle distance.

Have you checked the Z axis? Maybe the screws are getting out of sync due to binding, slipping, etc.

The z is a bit loud, and I was thinking of lubricating it a bit with machine oil, and the rods were never completely straight, but measuring the distances the sides corresponding to the 2 different motors travel, they seem to be the same. So I don't think it's binding or slipping on the z, though I can try watching it more carefully.

Check the couplers that connect the Z axis screws to the motors. If one or both are slipping the Z axis will never behave properly. Make sure the motors have adequate current.

Did check all the couplers: all tight, and z axis shows no slipping as far as I can tell. One of the rods is less straight than the other though, don't know if that could be playing a role (it squeaks a bit more in certain regions too). Can't see any binding either during movement.

There was a bit of a lean (probably 3-4 degrees at most) to the y axis guides vs the rest of the frame, not sure if that would cause misalignment of nozzle and bed unless there was slipping/binding going on.

I'll try a few more prints and see if it keeps doing that. I don't see what could be the problem besides a wobbly bed or the z screws. Maybe printing a tall hollow tower and measuring its base to wall angle to see if the bed and z are perpendicular is a good idea.

Checking alignment or axes is best done printing a rectangular solid. A rectangular solid will have two diagonals in each plane. You can check all three axes for orthogonality by measuring diagonals in the XY, YZ, and XZ planes.

Leveling the bed does not compensate for axes that are not orthogonal. It only lets the first layer stick to the bed. If the axes are not orthogonal circles won't be circular and you'll have trouble printing with any precision, especially parts that have to fit together (tight fitting lids for boxes) or mesh (like gears or threads).

Well, it seems that my circles are ok and prints come out fine. My protractor on some rectangular parts tells me that the axes are perpendicular or nearly so.

It's mostly the constant releveling issue that's been bothering me, so I was wondering if replcaing the melamine bed by an aluminum one might help. Now that I know the z screws are orthogonal to the bed, the only correction I can think of to the z axis is actually replacing said screws.

Today's printing seemed to better in terms of the Z homing consistency, I guess I'll keep watching it to see if the problem comes back.

If the problem is at the bed and not the X axis, it's probably a structural problem, not the material from which the bed is made. A lot of printers are relatively flexible and after you apply some effort to remove a well-attached print the bed ends up in a different position than it was before, requiring releveling/zeroing. The cure for that is either beef up the structure so it doesn't happen or add autotramming so you don't have to deal with it manually.

I think replacing the wood with rigid aluminum could count as beefing up the structure. In fact, I don't see any other way to do it in the case of the prusa i3, as the entire thing consists of the bed plate and the heated bed riding on top of it on some springs. Correct me if I'm wrong.

It does seem like Z homing is now fairly consistent, not sure what I'm doing right, perhaps the melamine frame settled into its preferred shape and is staying there. If it starts happening again though, from our discussion it seems like a bed structure reinforcement is the only real option.

Autoleveling sounds nice, I was just hoping to avoid the extra work of putting that in if I could.

Once you've got auto bed leveling going it saves you a whole lot of repeated adjusting the bed. It's also a handy tool to measure the bed flatness and for adjusting the Z height offset digitally instead of using the screw on the x motor end. If you have the EEPROM enabled and a display you can set the z offset from the menu control/motion. I don't have much problems with the wood structure though.

The way I have it now is that I've used metal washers to level the heated bed perpendicular with the frame (I used a 20x20cm aluminium place for reference). I don't use any springs and place the 2mm thick glass plate directly on top of the heated bed. It's very solid and sturdy, no play at all. I can now use acceleration of 2000mm/s2 with no shaking or noise. The auto bed leveling does the rest.
The y axis layers look cleaner than the x axis (Bowden setup) now, but that's due to the LM8UU bearings that are a little loose. Will be replacing them soon. Getting the stepper motor off the x carriage made a huge difference in vibrations and print speed/acceleration. On the cold/hotend part I have now bowden, 2 fans (one for cooling the coldend, one for cooling the print), a 9g servo for ABL. I'm planning to put a couple of white leds on the underside for print illumination. I've redesigned the hotend holder to include all these parts.

I just haven't explored leveling that much yet, there seem to be a few options available and it'd be nice to choose a good one. I'll definitely get it going at some point, but I need a printer that consistently prints well in the meantime...

Looks like I'm getting a new bed plate after all. Wooden one broke today: not sure if it was the pressure or from the filament roll falling on it (printer shakes way too much) or a combination of both, but I'll have to replace it now. Will see if the aluminum one works better.

I like the bowden concept, but a bit worried about jamming since it can't push as efficiently. I can definitely see how it would make for nicer prints, probably will try it out when my printer's actually working again.